It is well known that the presence or the properties of substances on a material's surface can be determined by light-based sensors. Polarization-based techniques are particularly sensitive; ellipsometry, for example, is a widely used technique for surface analysis and has successfully been employed for detecting attachment of proteins and smaller molecules to a surface. In U.S. Pat. No. 4,508,832 to Carter, et al. (1985), an ellipsometer is employed to measure antibody-antigen attachment in an immunoassay on a test surface. Recently, imaging ellipsometry has been demonstrated, using a light source to illuminate an entire surface and employing a two-dimensional array for detection, thus measuring the surface properties for each point of the entire surface in parallel(G. Jin, R. Janson and H. Arwin, “Imaging Ellipsometry Revisited: Developments for Visualization of Thin Transparent Layers on Silicon Substrates,” Review of Scientific Instruments, 67(8), 2930-2936, 1996). Imaging methods are advantageous in contrast to methods performing multiple single-point measurements using a scanning method, because the status of each point of the surface is acquired simultaneously, whereas the scanning process takes a considerable amount of time (for example, some minutes), and creates a time lag between individual point measurements. For performing measurements where dynamic changes of the surface properties occur in different locations, a time lag between measurements makes it difficult or impossible to acquire the status of the entire surface at any given time. Reported applications of imaging ellipsometry were performed on a silicon surface, with the light employed for the measurement passing through+the surrounding medium, either air or a liquid contained in a cuvette. For applications where the optical properties of the surrounding medium can change during the measurement process, passing light through the medium is disadvantageous because it introduces a disturbance of the measurement.
By using an optically transparent substrate, this problem can be overcome using the principle of total internal reflection (TIR), where both the illuminating light and the reflected light pass through the substrate. In TIR, the light interacting with the substance on the surface is confined to a very thin region above the surface, the so-called evanescent field. This provides a very high contrast readout, because influences of the surrounding medium are considerably reduced. In U.S. Pat. No. 5,483,346 to Butzer, (1996) the use of polarization for detecting and analyzing substances on a transparent material's surface using TIR is described. In the system described by Butzer, however, the light undergoes multiple internal reflections before being analyzed, making it difficult or impossible to perform an imaging technique, because it cannot distinguish which of the multiple reflections caused the local polarization change detected in the respective parts of the emerging light beam. U.S. Pat. No. 5,633,724 to King, et al. (1997) describes the readout of a biochemical array using the evanescent field. This patent focuses on fluorescent assays, using the evanescent field to excite fluorescent markers attached to the substances to be detected and analyzed. The attachment of fluorescent markers or other molecular tags to the substances to be detected on the surface requires an additional step in performing the measurement, which is not required in the current invention. The patent further describes use of a resonant cavity to provide on an evanescent field for exciting analytes.